Lithium (Li) ion electrochemical cells typically have a high energy density and are commonly used in a variety of applications which include consumer electronics, wearable computing devices, mobile equipment, satellite communication, spacecraft devices and electric vehicles. Lithium ion cells are particularly popular for use in large-scale energy applications such as low-emission electric vehicles, renewable power plants and stationary electric grids. Additionally, lithium ion cells are at the forefront of new generation wireless and portable communication applications. One or more lithium ion cells may be used to configure a battery that serves as the power source for these applications. The explosion in the number of higher energy demanding applications and the limitations of existing lithium ion technology are accelerating research for higher energy density, higher power density, higher-rate charge-discharge capability, and longer cycle life lithium ion cells. Today's commercialized lithium ion cells employ lithium intercalation materials for both the cathode and the anode.
Lithium ion cells are mainly composed of an anode, for example, graphite, a carbonate-based organic electrolyte, and a cathode comprising a cathode active material, for example, lithium cobalt oxide (LiCoO2). Lithium ions are intercalated and deintercalated between the anode and the cathode through the electrolyte during discharge and charge. When the cell supplies power, or is discharging, lithium ions generally move from the negative electrode (anode) to the positive electrode (cathode). When the cell is storing energy for later use, or is charging, the opposite occurs. Lithium ions generally move from the positive electrode (the cathode) to the negative electrode (the anode) during charging. For the example, the theoretical capacities of a graphite anode and a LiCoO2 cathode are about 372 mAh/g and less than about 160 mAh/g, respectively. These theoretical charge capacities, however, are too low for the recent surge in higher energy demanding applications.
Since it was first demonstrated that lithium metal can electrochemically alloy with other metals at room temperature, lithium alloying reactions with metallic or semi-metallic elements and various compounds have been investigated during the past few decades. Of the various lithium alloying elements studied for use in lithium ion cells, silicon (Si) has been considered one of the most attractive anode materials, because of its high gravimetric and volumetric capacity, and because of its abundance, cost effectiveness, and environmentally benign properties.
Increasing the specific capacity of anodes of lithium ion batteries through the substitution of graphite has tremendously influenced the direction of recent scientific efforts. It is well known, however, that the commercial graphite anode cannot meet these challenges due to its low theoretical capacity (372 mAh/g). Silicon, the second most abundant chemical element on earth, has a theoretical capacity of about 3,572 mAh/g, almost 10 times the capacity of graphite. Thus, there is a consensus that a breakthrough in capacity can be achieved by moving from classical intercalation reaction to an alloying reaction because the alloying reaction can store more lithium compared with intercalation reaction. Utilization of silicon offers the potential for a high capacity lithium alloying reaction capable of producing a lithium-rich phase (e.g. Li15Si4 and Li22Si5) compared with an intercalation reaction with graphite (LiC6). However, there are still some areas for improvement. For example, the increased accommodation of Li+ ions during charge-discharge cycles induces large volume variations (as much as about 370%) and stress on a bulk anode matrix that may ultimately shorten the useful life of the anode. Hence, different options have been pursued to alleviate the effect of volume expansion including the use of amorphous thin films, nanowires, nanotubes, and porous morphologies. Despite these advances, a significant capacity degradation during charge-discharge cycles is still observed. This suggests electrode fracturing which eventually leads to electrical contact losses. To address this specific issue, a promising anode material comprising a graphene-composite material in a graphite network was developed by one of the inventors of the embodiments of the present invention. The material is comprised of a continuous network of graphite regions integrated with, and in good electrical contact with a composite comprising graphene sheets and silicon, an electrically active material, wherein the electrically active material is dispersed between, and supported by, the graphene sheets.
Another existing limitation, however, is that the technology suffers from fast capacity fading. Fast capacity fading greatly hampers the application of silicon anode materials. Capacity fade is generally attributed to initial energy losses due to lithium ion consumption generated by side reactions on the active anode material surface. Hence there is a need to minimize these initial losses so that the available energy can be substantially improved.
Various forms of silicon electrode materials have been tested, including silicon particles mixed with a binder and conducting carbon, nanowires, thin films, and 3-dimensional porous particles. However, these are still not satisfactory, either because of poor cycling stability, cost of manufacturing, and/or insufficient capacity improvement. Although the exact causes for storage capacity loss upon cycling are still under investigation, various attempts to stabilize these structures have been reported. The most common approach is to encapsulate the silicon structure with a conducting carbonaceous layer, in hope that this would better retain the silicon fragments from being disconnected from the conducting electrode. Various precursors can be used for encapsulation, including resorcinol-formaldehyde gel, poly(vinyl chloride)-co-vinyl acetate or polyvinyl chloride and chlorinated polyethylene, glucose, and fullerene (C60). Noticeable improvements were achieved, but capacity degradation was not eliminated.